Since polyesters such as polyethylene terephthalate (PET) have excellent chemical stability, they have been used for fibers, films, sheets, beverages bottles, etc. Since unsaturated polyesters are lightweight and excellent in weathering resistance, chemical resistance and heat resistance, they have been used as fiber-reinforced plastics (FRP) for producing molded articles, such as purifying tanks, bathtubs and small ships.
In recent years, disposal of waste matters of the above polyesters and the like has become a problem, and various methods to recover and reuse the waste matters have been studied. As one of the methods, so-called feedstock recycling comprising depolymerizing a waste matter of a polyester or the like to convert it into monomers and performing polymerization reaction using the monomers as raw materials to form a polyester such as PET again has been studied. This feedstock recycling is expected as a means capable of realizing reuse of resources because separation of impurities is feasible and the qualities of the raw materials do not differ so much from those of virgin materials.
As methods for depolymerizing a polyester into monomers, broadly divided three methods, i.e., a hydrolysis method using water as a solvent, an alcoholysis method using an alcohol as a solvent and a glycolysis method using a glycol as a solvent, have been proposed.
The hydrolysis method is, for example, a method wherein a polyethylene terephthalate melt is allowed to react with water vapors and then allowed to react with ammonium hydroxide to decompose polyethylene terephthalate into terephthalic acid and ethylene glycol (patent document 1). Although this method has an advantage that a glycol or an alcohol does not need to be used for the reaction, it is necessary to use a pressure-resistant special apparatus because the reaction is carried out under the conditions of high pressure.
The alcoholysis method is, for example, a method wherein a polyester is heated in an alcohol solvent (if necessary, a catalyst is added) to depolymerize the polyester (patent document 2, patent document 3). This method has an advantage that when PET is depolymerized using, for example, methanol as a solvent, dimethyl terephthalate (sometimes abbreviated to “DMT” hereinafter) that is a useful and easy-handling monomer is directly formed by the depolymerization reaction and the depolymerization reaction proceeds relatively rapidly. On the other hand, the alcohol used as a solvent is low-boiling, and in order to promote the reaction, application of pressure is necessary (for example, reaction is carried out in methanol in a supercritical or subcritical state), so that there is a problem that a pressure-resistant special apparatus is necessary.
The glycolysis method is a method wherein a polyester is heated together with a depolymerization catalyst such as sodium carbonate in an excess alkylene glycol solvent to depolymerize the polyester and thereby form a bis(β-hydroxyalkyl)terephthalate and ethylene glycol (patent document 4, patent document 5). For example, when ethylene glycol is used as a solvent, bis(β-hydroxyethyl)terephthalate (sometimes abbreviated to “BHET” hereinafter) is formed by the depolymerization reaction, and by further adding methanol in the presence of a transesterification catalyst and performing transesterification reaction, DMT can be recovered.
In this glycolysis method, the reaction can be carried out at atmospheric pressure. However, the reaction time is relatively long (e.g., about 4 hours according to the working example of the patent document 4), and hence, operating efficiency is not enhanced. Further, there is another problem that the glycol as a solvent is deteriorated because it is heated for a long period of time. Therefore, shortening of the reaction time has been desired.
In a patent document 6, a depolymerization method wherein PET is allowed to react with sodium hydroxide in ethylene glycol containing sodium hydroxide and water to form sodium terephthalate is disclosed. According to this depolymerization method, it is possible to depolymerize PET for a shorter period of time (about 15 minutes to 1 hour) than in the above-mentioned glycolysis method, and for example, in the working example of the patent document 6, it is described that by heating and stirring PET at about 180° C. for about 1 hour, PET was depolymerized and sodium terephthalate was obtained.
The glycolysis method is also utilized as a method for depolymerizing an unsaturated polyester. For example, in a patent document 7, a method for depolymerizing an unsaturated polyester comprising heating unsaturated polyester waste in a glycol solvent such as ethylene glycol (preferably in the presence of a catalyst such as sodium ethylate) is proposed. Also in the depolymerization of an unsaturated polyester by this method, however, a long period of time is required for the reaction, similarly to the case of depolymerizing a polyester. For example, in Example 1 of the patent document 7, it is described that reaction at 180° C. for 5 hours and then at 200° C. for 8 hours resulted in an unsaturated polyester decomposition ratio of 54.4%. However, the levels of this period of time required and the decomposition ratio are insufficient for industrialization.
In a patent document 8, a depolymerization method wherein unsaturated polyester waste is allowed to react with a glycol at a relatively high temperature of about 150 to 300° C. in the presence of a radical initiator to cut even a part of styrene crosslinkage that is hardly decomposed by a usual glycolysis method, and a method for synthesizing an unsaturated polyester by subjecting the above decomposition product (oligomer) to condensation reaction with a dibasic acid are disclosed. Also this depolymerization method, however, needs a high temperature, a high pressure and a long period of time, and for example, in Example 1 of the patent document 8, it is described that reaction was carried out at 290° C. for 2 hours and that the pressure at the time of completion of decomposition was 3.7 MPa.
In a patent document 9, it is described that in a step of depolymerizing a polyester into an oligomer (e.g., degree of polymerization: not more than 800), the depolymerization reaction is accelerated by irradiation with microwaves using an electronic oven or the like and that also in a step of producing an unsaturated polyester resin by adding an unsaturated polybasic acid or the like to the oligomer obtained by the above depolymerization and then copolymerizing them, the polymerization reaction is accelerated by irradiation with microwaves. Further, it is suggested that microwaves not only increase the temperature of the reactant in the polymerization reaction (esterification reaction) or the depolymerization reaction but also exert a great effect on the reaction itself.
More specifically, it is described in Example B of the patent document 9 that waste PET bottle flakes, propylene glycol and dibutyl oxide that was a depolymerization catalyst were introduced into an extruder first, the PET was depolymerized to prepare an oligomer having a molecular weight of about 1500, and then the oligomer was irradiated with microwaves at a frequency of 2450 MHz for 30 minutes under heating at not lower than 180° C. with stirring to thereby decrease the molecular weight to about 700.
According to the invention described in the patent document 9, however, even if the polyester is depolymerized by continuously irradiating it with microwaves for about 30 minutes after heating, the resulting substance is an oligomer to the end, and any monomer favorable for reuse has not been obtained. Moreover, it is necessary to depolymerize the polyester to a certain extent using an extruder prior to irradiation with microwaves.
Patent document 1: JP 2003-527363 A
Patent document 2: JP H11(1999)-100336 A
Patent document 3: JP 2003-300916 A
Patent document 4: JP 2002-167468 A
Patent document 5: JP 2004-300115 A
Patent document 6: JP H11(1999)-302208 A
Patent document 7: JP H08 (1996)-225635 A
Patent document 8: JP 2005-255780 A
Patent document 9: JP 2003-292594 A
Such publicly known conventional methods for depolymerizing a polyester or an unsaturated polyester as described above require a long processing time of at least several tens of minutes to several hours, and this causes a bottleneck in the mass disposal of polyester waste. In the conventional methods, further, reaction must be carried out under the conditions of high temperature or high pressure, and therefore, a special apparatus capable of withstanding such reaction conditions has been also required. It is an object of the present invention to solve such problems in such conventional feedstock recycling.
That is to say, it is an object of the invention to provide a method for depolymerizing a polyester and an unsaturated polyester, which can be rapidly carried out by a simple apparatus. It is another object of the invention to provide, in another embodiment of the invention, a method for recovering a polyester monomer containing small amounts of impurities, which uses the depolymerization.